When evaluating the features and properties of a particular magnetic core structure, it is helpful to think in terms of the currents which flow in the windows of the structure and the corresponding electromagnetic response by the core structure. Any currents which flow in two parallel conductors can be described in terms of common mode and differential mode currents. Common mode currents are those which flow in the same direction in the two conductors, while differential currents flow in opposite directions in the conductors. Given this definition, arbitrary currents in two conductors can be described as a combination of common mode and differential mode currents. Both alternating currents, AC and direct currents, DC, can be described as common and differential mode.
All prior art core structures are able to accommodate certain types of currents, but have correspondingly poor performance with respect to other currents. For example, FIG. 1(a) depicts a conventional ungapped E--E magnetic core. This type of core is termed an E--E core because it is typically constructed from two symmetrical halves, each of which is shaped like the letter E. Such an ungapped E--E core provides high differential mode inductance, and high common mode inductance to currents passing through the windows of the E--E core. But such an ungapped core provides negligible direct current capability.
In order to provide dc capability, the center leg of a conventional E--E core can be gapped in a horizontal fashion at the mating surface as depicted in FIG. 1(b). The outer legs in the structure of FIG. 1(b) are left ungapped. With such a center leg gap, a conventional E--E core can accommodate differential dc current and provide common mode inductance, but at the expense of much reduced differential mode inductance.
The outer legs of a traditional E--E core can also be gapped in a horizontal fashion at the mating surfaces, as seen in FIG. 1(c). An E--E core with the outer legs gapped in such a manner can accommodate both common and differential mode dc current, but at the expense of much reduced common and differential mode inductance.
One further way to modify a conventional E--E core is to provide a vertical gap in the center leg as shown in FIG. 1(d). An E--E core with such a vertical gap will accommodate common mode dc current without a corresponding reduction of the differential mode inductance. However, a core with this type of vertical gap is limited in its range in that the common mode inductance can never be reduced to less than one quarter the differential mode inductance. Correspondingly, for a given differential mode inductance, there is an upper limit to the common mode dc current which can be accommodated, no matter how large the gap is made.
It is therefore an object to provide common mode inductance, differential mode inductance and common mode dc current capability in a single magnetic core structure.